This invention relates to optical fibre cables, particularly long span aerial cables incorporating optical fibres.
Various cable designs have been proposed for use as aerial cables incorporating optical fibres, for example as earth wires in an overhead power transmission system. When an overhead power transmission system is installed, it is convenient to use the same route for purely telecommunications purposes and it has already been proposed to provide a communication system via an earth wire of the power transmission system. British Patent No. 2029043 B is an example of an overhead earth wire cable for a power transmission system incorporating an optical fibre for telecommunication purposes.
On existing power transmission routes which have not had an optical fibre cable installed in the earth wire, then three alternatives exists in order to install a fibre optic cable in an existing route. The first alternative is to replace the existing standard earth conductor wire with a fibre optic earth wire as mentioned above in British Patent No. 2029043 B; another alternative would be to wrap a fibre optic cable around a power conductor of the system; and a third alternative would be to install a self-supporting optical fibre aerial cable by suspending it from the pylons which support the existing system.
The first two options above are expensive and inconvenient, requiring as they do the complete shutdown of the power transmission system while the modifications are effected.
The third alternative offers the more satisfactory solution. However it is undesirable to install a cable which contains metallic elements because the presence of an additional electrically conductive cable in the vicinity of the power conductors of a power transmission system adversely affects certain aspects of the existing system operation. it is therefore necessary to provide a non-metallic fibre optic cable and such a cable has already been proposed. This known aerial fibre-optic cable is made by Standard Electric Lorenz in Germany and comprises a helically-laid-up fibre optic package surrounded by a glass-fibre reinforced tube acting as the strength member and formed into position around the fibre optic bundle during manufacture of the glass fibre reinforced strength member.
Although such a cable is effective in providing a self-supporting telecommunications link in an existing power transmission system, it nevertheless has certain disadvantages, not least being the cost of the cable and the limited amount of excess fibre which can be achieved in order to minimise damage to the fibre under conditions of use.
It is an object of the present invention to provide a metal-free aerial optical fibre cable which is cheap to produce and effective in its application.